Turgor and the conformational pathway for MscS recovery

The bacterial mechanosensitive channel MscS is an adaptive osmolyte release valve that cycles between closed, open, and inactivated states. Since some of these conformations are stable only in the lipid environment under specific conditions, the structures that are currently available cannot explain the entire functional cycle. Previous patch-clamp characterization has provided insights into the missing functional state by estimating protein expansion areas associated with the closed-to-open and closed-to-inactivated transitions and indicating that the closed state must be the most compact. In this paper, we model the conformational transition of MscS from the splayed conformation with the uncoupled gate to the putative compact closed state. The compaction pathway revealed in preliminary extrapolated motion simulations (ExMoS) involved an upward sliding motion of the internal TM3 barrel inside the outer sheath formed by TM1-TM2 helical pairs. This move leads to several structural changes: (1) the relocation of the characteristic kink at G113 to a new position at G121, (2) the establishment of the hydrophobic TM2-TM3 contact, (3) a new pattern of interactions with membrane lipids, and (4) the formation of stabilizing salt bridges between TM1-TM2 loops and the cytoplasmic cage domain. In the intact bacterial cell, the driving force for this upward motion is likely to be turgor pressure normal to the plane of the membrane acting on the upper hemisphere of the cage domain from the inside. Under continuing lipid synthesis in the inner leaflet of the plasma membrane, turgor pressure is also predicted to maximize the lateral pressure of lipids in the membrane, thus driving MscS compaction. Steered simulations were performed on the splayed state to mimic these effects by applying normal forces to the upper part of the cage domain and by applying lateral compression to the TM1-TM2 pairs, emulating the pressure of lipids. The structure arrived at the predicted compact state of the channel. This state was critically stabilized by displacing non-bilayer lipids from the TM2-TM3 crevices into the bilayer. We propose that the energized metabolic state of the cell generating high turgor and promoting lipid synthesis should strongly favor the compact closed state of MscS. The normal forces pressing the dome of the cage domain against the membrane may provide a common recovery mechanism for the entire family of MscS-like channels found exclusively in organisms with walled cells, which evolved to function under turgor pressure. A conversion of turgor into membrane tension under hypoosmotic cytoplasm swelling and peptidoglycan expansion will drive opposite processes of opening followed by adaptive MscS closure and inactivation.